1. Field of the Invention
The present invention relates to a content receiver, a content reproducer, a content reproducing system, a content writing-out method, a viewing expiration time determining method, and the like for downloading rental content protected by copyright from a content server on a network, writing out the rental content to an exchangeable medium together with a viewing expiration time, and determining a viewing expiration time on the exchangeable medium with the content reproducer to determine possibility of reproduction of the content.
2. Background Art
In recent years, according to the development of the Internet, techniques for downloading and reproducing movies, music, and the like are generally used. A large number of services for downloading contents from servers, which perform content delivery of movies, music, and the like, to PCs, AV apparatuses, cellular phones, and portable apparatuses through networks are provided.
As forms of such content delivery services, there are content sales of a sell-off type with which a user can permanently view purchased content and rental sales with which a user can view downloaded content within a limited viewing expiration time.
In the typical content sales of a download type, a content provider which supplies content operates a content server and a DRM server and a content receiver on a viewer side downloads encrypted rental content from the content server and acquires a decryption key for the encrypted content and viewing conditions for the content from the DRM server. The viewing conditions for the content include information concerning a viewing expiration time in which a user is permitted to view the content and a copy condition for the content. When the content receiver reproduces the rental content, the content receiver refers to a viewing expiration time designated from the DRM server and, if the viewing expiration time has not expired, starts reproduction of the content and, if the viewing expiration time has expired, carries out deletion of the content. As a viewing expiration time managing method of this type, for example, there is a method described in Patent Document 1 (Japanese Patent Application Laid-Open No. 2003-288274).
A diagram of a conventional content delivery system for explaining the concept of a viewing expiration time managing method for rental content in conventional content download is shown in FIG. 4.
A content receiver 102 accesses, via a network (not shown), a content server 100 and a DRM server 101 operated by a content provider.
When the content receiver 102 carries out download of rental content separately designated by a user, the content receiver 102 downloads encrypted content 104 from the content server 100 and stores the content 104 in a storage medium 131 such as a built-in HDD and, at the same time, acquires a content key 105 for decryption of the encrypted content 104 and viewing conditions 130 such as a viewing expiration time for the content 104 from the DRM server 101.
When the content receiver 102 itself carries out reproduction of the content, a viewing expiration time determining unit 133 reads out present date and time information from a secure clock 134 and compares the present date and time information with the viewing expiration time included in the viewing conditions 130 acquired from the DRM server 101 to thereby determine whether the viewing expiration time for the content 104 has expired. Thereafter, if the viewing expiration time for the content 104 has not expired, the viewing expiration time determining unit 133 causes a DRM decryption unit 107 to decrypt the encrypted content 104 using the content key 105 and causes an output unit 132 to output the content 104 to reproduce the content 104.
In the following explanation, as a further developed form of the above, the content receiver 102 writes out, to an SD card 103, the content 104 as rental content received from the content server 100 and the user performs viewing in a content reproducer 119 separate from the content receiver 102.
The content receiver 102 decrypts, in the DRM decryption unit 107, the encrypted content 104 using the content key 105, carries out encryption again in a CPS encryption unit 108 in a CPRM (Contents Protection for Removable Media) format, which is a CPS (Contents Protection System) for an SD card, writes the content 104 in the SD card 103, and writes a CPS key 121 used for decryption of a CPS cipher in association with the content 104. A viewing condition converting unit 109 of the content receiver 102 converts the viewing expiration time included in the viewing conditions 130 of the content 104 received from the DRM server 101 into a CPS format and records the viewing expiration time in the SD card 103 as a CPS viewing expiration time 110 in association with the content 104.
The content reproducer 119 is a content viewing terminal having a function of reproducing an SD card recorded in the CPRM format.
When the content reproducer 119 reproduces the content 104 recorded on the SD card 103 inserted in the content reproducer 119, first, a viewing expiration time acquiring unit 140 reads out the CPS viewing expiration time 110 associated with the content 104 from the SD card 103 and passes the CPS viewing expiration time 110 to the viewing expiration time determining unit 113. When the viewing expiration time determining unit 113 receives the CPS viewing expiration time 110 from the viewing expiration time acquiring unit 140, the viewing expiration time determining unit 113 reads out time setting date and time 115 stored in advance and reads out the present date and time information from the secure clock 114. The time setting date and time 115 is date and time information at the time when time setting for the secure clock 114 is performed last time, i.e., data and time information at the time when update of time of the secure clock 114 is performed last. The time setting date and time 115 is stored in the content reproducer 119.
As date and time management in this secure clock 114, CPS time 117 acquired from a CPS time server 116, which supplies CPS time, via the network is set in the secure clock 114 to perform hand movement management. Since a clock of an apparatus has an error, time information set once cannot be trusted forever. A period in which the CPS time 117 acquired from the CPS time server 116 is trusted is set in advance. For example, when one week elapses after the CPS time 117 is acquired, it is determined that errors accumulate and time cannot be trusted. The CPS time 117 is acquired from the CPS time server 116 again. However, until one week elapses, date and time information of the secure clock 114 updated and subjected to hand movement management on the basis of the CPS time 117 acquired last time is trusted. The reacquisition of the CPS time 117 from the CPS time server 116 is not performed.
In this way, the viewing expiration time determining unit 113 determines on the basis of the date and time information acquired from the secure clock 114 whether the CPS viewing expiration time 110 has not expired. When the CPS viewing expiration time 110 has not expired, the viewing expiration time determining unit 113 notifies a reproduction control unit 120 that reproduction of the content 104 is possible.
A CPS decryption unit 111 receives the notification that the reproduction of the content 104 is possible from the reproduction control unit 120, reads out the content 104 from the SD card 103, and outputs the content 104 of a plain text obtained by decrypting the content 104 with the CPS key 121 to an output unit 112. The output unit 112 decodes the content 104 and outputs the content 104 as an AV signal.
All the disclosures of the Document 1 are cited in its entirety and thereby become an integral part hereof.
However, in the related art, when content, a viewing expiration time for which is managed in the copyright protection system, is written out to a replaceable recording medium (in this specification, referred to as “exchangeable medium”), management of the viewing expiration time corresponding to reliability of time management specified by the copyright protection system cannot be applied to the written-out content.
Specifically, in the content reproducer 119 of the conventional content delivery system, viewing expiration time determination is performed on the basis of the CPS time 117 acquired from the CPS time server 116 completely independent from time management in the DRM server operated when the original content 104 is supplied from the content provider.
When the content provider enters into an agreement for content procurement from a content right holder, i.e., a movie production studio, usually, after examining in detail technical requirements in the content delivery system as the copyright protection system, i.e., a DRM system, the content provider persuades the content right holder to consent to the reliability of a DRM technique and then obtains a content procurement license in content business.
From a technical viewpoint, it is within the assumption of the technical requirements, to which the content provider and the content right holder consent, that the content 104 as the rental content is viewed in the content receiver 102 on the basis of accuracy and reliability of time management of the DRM system including the DRM server 101. When the content 104 protected by the DRM is output, i.e., exported to the CPS as a content protection system other than the DRM used for delivery, the content 104 protected under the management of the DRM until being delivered to and stored in the content receiver 102 is protected under the management of the CPS rather than under the management of the DRM after being exported to such as a SD card. However, it is not always guaranteed that accuracy and reliability of time management in the CPS at an export destination are equivalent to or higher than those in the DRM. Therefore, it is likely that the content 104 is exported to a CPS with low reliability of time management. This indicates likelihood that viewing conditions, i.e., viewable time and the like intended by the content provider and the content right holder are not observed.
Specifically, when accuracy of the CPS time server 116 is low, there is a shift in time, or fault tolerance is low compared with time reliability of the DRM system intended by the content provider, there is a problem in that the content reproducer 119 can reproduce the content 104 exceeding a range of the viewing expiration time for the content 104 licensed to the content receiver 102 by the DRM server 101 or, conversely, although there is still time until a viewing expiration time originally designated, it is determined by mistake that the viewing expiration time has expired.
In view of the conventional problems, it is an object of the present invention to provide a content receiver, a content reproducer, a content reproducing system, a content writing-out method, a viewing expiration time determining method, and the like which can apply, when content, a viewing expiration time for which is managed in a copyright protection system, is written out to an exchangeable medium, management of the viewing expiration time corresponding to reliability of time management specified by the copyright protection system even to the written-out content.